1. Field of the invention
The present invention relates to an electrophotographing apparatus. More specifically, the present invention relates to an electrophotographing apparatus such as a copying machine, facsimile and etc., in which an electrostatic latent image is formed on a photoreceptor by exposing the photoreceptor, and then, developed with a toner.
2. Description of the prior art
A copying machine 1 that is one example of such a kind of prior art is shown in FIG. 7. In the copying machine 1, a light from a light source 6 is irradiated onto an original 5 put on an original table 4, and a photoreceptor 2 is exposed by a reflected light from the original 5, and therefore, an electrostatic latent image according to an original image is formed on the photoreceptor 2, and thereafter, the electrostatic latent image is toner-developed by a developer 3 to which a developing bias voltage Vb is applied.
At this time, in a case where the original 5 which is put on the original table 4 is an original having a low reflectance, such as a newspaper, a light amount of an exposure light irradiated onto the photoreceptor 2 is decreased, and therefore, an exposed voltage VL on the photoreceptor 2 becomes to be lowered, and accordingly, a background occurs if no countermeasure is taken.
In the prior art, in order to suppress the background, the light amount of the exposure light irradiated onto the photoreceptor 2 is kept constant by increasing the light amount of the light source 6 for exposing the original 5, so that the decrease of the exposed voltage VL is suppressed. More specifically, a reflectance of the original 5 is sensed or detected by a photosensor 7 such as a photodiode and etc., and a voltage applied to the light source 6, i.e. a voltage outputted from an AVR (Automatic Voltage Regurator) is changed in response to an output of the photosensor 7, whereby the light amount for exposing the original 5 can be changed so as to keep the light amount of the exposure light constant. In addition, if a developing bias voltage Vb applied to the developer 3 is made lower according to a drop of the exposed voltage VL, it is possible to prevent the background from occurring. In either case, an electric conductive substrate 2a of the photoreceptor 2 is connected to the ground.
Now, with referring to FIG. 8(A)-FIG. 8(C), the above described prior art is more specifically described. A voltage model at a time that a normal original having a white background is put on the original table 4 is shown in FIG. 8(A), and a voltage model at a time that an original having a background with lower reflectance is put on the original table 4 and no correction is taken for the light amount of the exposure light is shown in FIG. 8(B). It is understood that in a case of the original having a low reflectance shown FIG. 8(B), the light amount of the exposure light irradiated into the photoreceptor 2 is decreased, and therefore, the exposed voltage VL is dropped to -120 V from -40 V of a case of the original having a normal reflectance (FIG. 8(A)). In this example, because the developing bias voltage Vb is -140 V, the voltage difference between the developing bias voltage Vb and the exposed voltage VL is decreased to 20 V from 100 V, and therefore, the above described background occurs. Then, as shown in FIG. 8(C), the exposed voltage VL is kept at -40 V approximately by increasing the light amount of the light source 6 for exposing the original 5, or the developing bias voltage Vb is decreased to -280 V approximately, and therefore, the voltage difference between the developing bias Vb and the exposed voltage VL is kept at 100 V approximately such that the background can be suppressed.
However, if it is intended to suppress the background by suppressing the decrease of the exposed voltage VL by increasing the light amount of the light source 6 for exposing the original 5, a dark voltage Vd that is equal to an image portion of the original 5 is increased due to the increase of the light amount. More specifically, the dark voltage Vd is increased to -490 V from -540 V as shown in FIG. 8(A) and FIG. 8(B). Therefore, a voltage difference between the developing bias voltage Vb and the dark voltage Vd becomes small, and therefore, an image density is lowered. A similar or the same disadvantage occurs in a case where the background is suppressed by decreasing the developing bias voltage Vb.